1. Field of the Invention
The present disclosure relates generally to input/output processing, and in particular, to insertion of multiple cyclic redundancy check words in an output data stream in an input/output processing system.
2. Description of Background
Input/output (I/O) operations are used to transfer data between memory and I/O devices of an I/O processing system. Specifically, data is written from memory to one or more I/O devices, and data is read from one or more I/O devices to memory by executing I/O operations.
To facilitate processing of I/O operations, an I/O subsystem of the I/O processing system is employed. The I/O subsystem is coupled to main memory and the I/O devices of the I/O processing system and directs the flow of information between memory and the I/O devices. One example of an I/O subsystem is a channel subsystem. The channel subsystem uses channel paths as communications media. Each channel path includes a channel coupled to a control unit, the control unit being further coupled to one or more I/O devices.
The channel subsystem may employ channel command words (CCWs) to transfer data between the I/O devices and memory. A CCW specifies the command to be executed. For commands initiating certain I/O operations, the CCW designates the memory area associated with the operation, the action to be taken whenever a transfer to or from the area is completed, and other options.
During I/O processing, a list of CCWs is fetched from memory by a channel. The channel parses each command from the list of CCWs and forwards a number of the commands, each command in its own entity, to a control unit coupled to the channel. The control unit then processes the commands. The channel tracks the state of each command and controls when the next set of commands are to be sent to the control unit for processing. The channel ensures that each command is sent to the control unit in its own entity. Further, the channel infers certain information associated with processing the response from the control unit for each command.
Performing I/O processing on a per CCW basis may involve a large amount of processing overhead for the channel subsystem, as the channels parse CCWs, track state information, and react to responses from the control units. Therefore, it may be beneficial to shift much of the processing burden associated with interpreting and managing CCW and state information from the channel subsystem to the control units. Simplifying the role of channels in communicating between the control units and an operating system in the I/O processing system may increase communication throughput as less handshaking is performed. However, altering command sequences, as well as roles of the channel subsystem and the control units, can cause difficulties in verifying the integrity of large quantities of data exchanged.
Existing communication protocols attempt to append a cyclic redundancy check (CRC) to the end of a message, allowing the receiver to verify message contents prior to taking action in response to the message. In order to fully verify the contents of the message using this approach, a large amount of buffer space may be needed at the receiver to temporarily hold the message until message verification is performed. Therefore, the size of available buffer space at the receiver represents an upper limit to the message size. Presently, if a message is larger than the available buffer size, the message may be subdivided into smaller messages that will fit into the available buffer size. This approach reduces communication and overall I/O processing system throughput, as additional overhead and delays are associated with sending numerous smaller messages. For example, sending multiple smaller messages may require extra support messages to establish locations for writing data in the smaller messages. It would be beneficial to develop an approach to insert verification checkpoints in an output data stream such that a message, which is larger than the available buffer size, can be incrementally verified. Such an approach would avoid the additional overhead and delays associated with subdividing a large message into multiple smaller messages. Accordingly, there is a need in the art for insertion of multiple CRCs in an output data stream in an I/O processing system.